1. Field of the Invention
The present invention relates to a laminated material, and more particularly, to a laminated material comprising an aluminum or polypropylene substrate and an ethylene copolymer resin composition layer firmly bonded to the substrate.
2. Description of the Prior Art
Aluminum foil or plate and polypropylene film or sheet are in widespread use as, for example, wrapping materials and industrial materials. These aluminum or polypropylene materials, however, are limited in their usefulness because they are not always satisfactory in all respects of necessary properties such as strength, heat resistance, water resistance, chemical resistance, gas permeability, printing properties, etc. One or more materials, therefore, have been laminated on aluminum or polypropylene to improve their properties so that they can be utilized in many industrial applications.
This lamination can be performed, for example, by using a solution type adhesive. This method, however, has many problems resulting from the use of solvents, such as bad working conditions and complicated process steps. With the advance in the automatic charge-wrapping system of, e.g., foods, drinks, etc., a method using a hot melt type adhesive is used in place of the aforenoted solution type adhesive. Ethylene-based copolymers such as an ethylene-vinyl acetate copolymer, etc. have been most widely used as the adhesive in such hot melt type adhesive processes. These ethylene-based copolymers, however, have insufficient adhesive strength and, therefore, it is necessary to use higher temperature and pressure when bonding or to physically or chemically treat the surface of the aluminum or polypropylene substrate, or to use supplementary adhesives.
In order to overcome the above-described problems of ethylene-based copolymer adhesives, it has been proposed to mix a modified polyolefin prepared by treating polyolefin with an unsaturated carboxylic acid or a derivative, or a resin which increases adhesion, or a low molecular weight resin, or the like with the ethylene-based copolymer. For example, known laminated materials include a composition consisting of an ethylene-vinyl acetate copolymer and a modified polypropylene, and a nylon substrate (Japanese Patent Publication No. 11032/1978), a laminated material comprising a composition consisting of low density polyethylene and a modified polypropylene, and a nylon (Japanese Patent Application Laid-Open No. 26548/1977), and a laminated material prepared by providing a layer of a composition consisting of a modified polypropylene, unmodified polypropylene and low density polyethylene or an ethylene-vinyl acetate copolymer on an aluminum plate (Japanese Patent Application Laid-Open No. 21850/1981).
The addition of such modified polyolefins or resins which improve adhesion or low molecular weight resins, however, fails to overcome the problems of ethylene-based copolymers. For example, a composition consisting of an ethylene-based copolymer and a modified ethylene-based polymer does not have sufficient high adhesion properties, and a composition containing resin that improves adhesion or a low molecular weight resin has insufficient heat resistance. Thus, laminated materials prepared using such compositions have only limited applications. It is further known that processes for lamination of such compositions containing an ethylene-based copolymer with another material required special procedures, and that the adhesive strength obtained may be insufficient depending on the material used. Therefore, the choice of an optimum material is performed by trial and error. Laminated materials prepared using a nylon resin can be used as wrapping materials, particularly in applications where ordinary storage and rot-proof properties are required, due to the excellent gas barrier properties of nylon, However, they are not suitable for use in applications where moisture-absorption must be prevented.
When laminating an ethylene-based copolymer resin composition layer on a metallic material such as aluminum or a polypropylene substrate, it is necessary to apply a preliminary treatment as described hereinbefore since the adhesion between the metallic material or polypropylene substrate and the ethylene-based copolymer resin composition layer is insufficient. In addition, when an extrusion lamination method is employed, it is essential that the metallic material or polypropylene substrate is coated with an anchor coating (priming) agent or is preliminarily heated. Particularly, in the case of the polypropylene substrate, there has been employed a method generally called "dry lamination" in which an adhesive is dissolved in, e.g., an organic solvent and is coated on both the polypropylene substrate and the other resin layer, which are then laminated together, since there are almost no resins exhibiting good adhesion to polypropylene. This method, however, suffers from problems such as scatter of solvent. Moreover, in the lamination of a resin composition layer consisting of an ethylene-based copolymer and modified polypropylene on an aluminum substrate, the relationship between the proportions of components in the composition and adhesion properties has not been determined.